When applying a scanning technique, a particle beam is magnetically scanned over a target. By varying the energy of the particle beam, different depths in the target volume can be reached. In this way, particle radiation dose can be delivered to the entire 3D target volume.
In current practice, irradiation planning devices for scanning techniques divide the delivery of the beam to a target in mono-energetic sections or layers.
The irradiation planning device defines the number of layers to be irradiated and for each layer a corresponding beam energy is defined. For each layer, the planning device typically defines a number of spots wherein each spot is defined by a dose D(i) and a beam position defined by coordinates X(i), Y(i). The coordinates correspond to the coordinates of the beam position in a plane perpendicular to the un-scanned beam.
In FIG. 3, a diagram is shown indicating the steps performed by well known irradiation planning devices and beam delivery systems. For such known systems, in step A, a number of spots are planned and the spots are grouped in a number of layers wherein each layer comprises spots of equal energy. In step B the plan is transmitted to the control system of the beam delivery system. Further, when performing an irradiation in step C, the output energy of the particle beam generator and the electromagnets of the beam transport system are set according to the energy of a first layer. Then step D is performed after step C is completed. In step D, the scanning magnets are set to position the beam to a first planned position X(1), Y(1) corresponding to the first spot of the first layer. Then in a step E, performed after completion of step D, the beam is turned on to deliver the planned dose to the first spot of the first layer. Thereafter, as an iterative process, step D and step E are repeated until all spots of the first layer have been irradiated. Finally, one restarts with step C going to the next layer and one iterates until all layers have been irradiated. In other words, in this scenario, the control of the variation of the output energy of the beam generator and control of the variation of the magnetic field of the scanning magnets is performed in series.
With the currently known systems, typically, the beam delivery system will follow a spot order that corresponds to for example a raster scan, wherein the beam is scanned for example from left to right and up and down. So with the well known devices, it is a scanning pattern that will define the order of the spots to be irradiated. There is also no specific order of the layers defined or imposed, the beam delivery system can execute the irradiation of the layers in any order.
This change of energy in-between layers takes some precious time during which no irradiation can be performed. The reason this energy change takes considerable time is related to a number of causes such as the inertia of mechanical devices that have to be moved as a function of energy (for example an energy degrader), the power supplies for the electromagnets of the beam transport system have limits in terms of how fast the currents can be changed, the magnetic field in the beam transport system is influenced by eddy currents impacting the time to settle the required magnetic field in the beam line, when superconducting electromagnets are used currents can only be changed slowly to avoid quenching. Furthermore, the reduction of time required to change energy is expensive due to the fact that devices with significant inertia (both mechanical devices and currents in electromagnets) need to be adapted.